std: Ensure fs::{DirEntry, ReadDir} are Send/Sync

[alexcrichton]

The windows/unix modules were currently inconsistent about the traits being
implemented for DirEntry and there isn't much particular reason why the traits
couldn't be implemented for ReadDir and DirEntry, so this commit ensures
that they are implemented.

Closes #22577

[rust-highfive]

r? @aturon

(rust_highfive has picked a reviewer for you, use r? to override)

[nagisa]

There is a good reason to not implement them. Citing the linux man pages:

The data returned by readdir() may be overwritten by subsequent calls to readdir() for the same directory stream.

This opens doors to memory safety issues if a race condition appears. This means that we either have use a Mutex and sacrifice single-thread performance, or not implement Sync.

[nagisa]

Looking at Windows’ implementation, I don’t see any synchronisation on a shared mutable data structure wfd either.
Nope, it is not shared, my bad.

[alexcrichton]

There is a good reason to not implement them. Citing the linux man pages:

The data returned by readdir() may be overwritten by subsequent calls to readdir() for the same directory stream.

This is quite true! It is also why we do not use readdir :). We use readdir_r where we pass in our own allocated buffer which is then owned by the DirEntry. This means that, like windows, each DirEntry owns its contents and is completely isolated from all others (hence my believe it grants Send and Sync)

[nagisa]

Ah +1 then.

[aturon]

@bors: r+ 756210a